The DAC4800A and Cherry amplifiers are "switching amps", so technically, the internal slew rate is that of the output stage (about 7200V/uS). The output filter of the amp removes the switching wave and yields an output slew rate of about 20V/uS (with an 8 ohm load). This is a respectable number, but keep in mind that it is limited by the output filter and increases as load impedance goes down.
The frequency response goes up to 60KHz on the DAC4800A and Cherry amps. This is a more valuable number than slew rate to many.
The sound of these amps can't be summed up with measurements (even though they are excellent) since they are designed from the ground up to deliver the best in music reproduction, not just test results in a lab.
Wednesday, October 8, 2008
Tuesday, September 30, 2008
Line Conditioners?
The DAC4800A and Cherry are designed to tolerate large AC mains fluctuations so you shouldn't need any line conditioning (save some money there). In fact, these amps even tolerate brief brown outs, playing through them without any noticeable effect.
Thursday, September 25, 2008
About Speaker Cables --- They Don't Need to Cost a Fortune!
It comes down to resistance and inductance. If you can solder, buy 12 gauge or 14 gauge (if the run length is less than 15 feet) wire and get terminations from Digi-Key. By terminations, I mean banana plugs or spade lugs. I definitely recommend soldered connections instead of crimped for long life. I agree that this is a typical area for spending too much when it really doesn't make a difference to the sound or your system. This comes from an engineer with 20 years experience, not a marketing firm.
Audio Review Still Hasn't Responded
We tried to add the DAC4800A amplifier (see http://www.digitalamp.com/4800a_amp1.htm) to Audio Review so we can get customers to add reviews there. However, there is no selection for adding a new manufacturer. The listing shows up with "unknown" as the manufacturer. Here's the link:
http://www.audioreview.com/cat/amplification/amplifiers/unknown/dac4800a/PRD_419903_1583crx.aspx
We wrote to them, but no reply. We posted to the help board, but the post was removed.
Does anyone know somebody in charge at Audio Review? We would like to get on the manufacturers list. Locking us out is just not fair. Thanks.
http://www.audioreview.com/cat/amplification/amplifiers/unknown/dac4800a/PRD_419903_1583crx.aspx
We wrote to them, but no reply. We posted to the help board, but the post was removed.
Does anyone know somebody in charge at Audio Review? We would like to get on the manufacturers list. Locking us out is just not fair. Thanks.
Why Our Amps Sound Better
Some of our customers have told us that they love the sound of our digital (technically "Class-D") amps, but they don't like the sound of other digital amps (from other companies). It might be the output filter or feedback design. People often criticize me for saying that feedback wrapped around the output filter of a Class-D amp can have a negative impact on the sound. Actually, this could be your answer!
By the way, fake class designations like "Class-T" or "Class-Z" are actually Class-D. The definition of "Class-D amplifier" is "an amplifier that uses its output devices as switches". This is a pet peeve of mine.
Thanks for hearing me out.
By the way, fake class designations like "Class-T" or "Class-Z" are actually Class-D. The definition of "Class-D amplifier" is "an amplifier that uses its output devices as switches". This is a pet peeve of mine.
Thanks for hearing me out.
Wednesday, September 24, 2008
Switching Amplifier Output Ripple Doesn't "Fold Down"
Regarding "folding down", this is called aliasing and is explained by sampling theory. Here is a basic explanation:
http://en.wikipedia.org/wiki/Nyquist%E2%80%93Shannon_sampling_theorem#Aliasing
Aliasing only happens when the waveform is sampled, which is not the case for a switching amplifier output driving a loudspeaker. The time domain output waveform of a switching amplifier (after the output filter) has switching ripple superimposed on top of the audio. The ripple is ultrasonic, so it is not heard. The mechanical limitations of loudspeakers do not allow this superimposed waveform to enter the room, not that it would matter anyway, as long as the switching frequency (Fsw) is high enough (>100KHz). Most switching amps change states (switch) at a rate of 350KHz or greater. Higher switching rates cause additional EMI and lower efficiency, and lower rates are harder to filter. The power dissipated in the loudspeaker by the ripple is typically a few milliwatts.
In order to accurately measure the performance of a switching amplifier, a filter that removes most of the ripple is usually placed between the amplifier under test and the measurement device. Such a filter requires flat response out to 20KHz or greater, but a rapid fall-off by the 100KHz range. Attempting to measure a switching amplifier without such a filter can drive the measurement equipment nuts as it tries to sync to the amplifier's output ripple instead of the audio signal.
Links to DAC4800A and Cherry Amplifier Information
Here's a link to the DAC4800A page:
http://www.DigitalAmp.com/4800a_amp1.htm
Here's a link to the Cherry page:
http://www.DigitalAmp.com/Cherry_amp2.htm
Here are links to reviews:
http://www.DigitalAmp.com/DAC_REVIEWS.htm
http://www.TrueDigitalPower.com/ (Amazon)
http://www.TrueDigitalAmp.com/ (Amazon)
http://www.DigitalAmp.com/4800a_amp1.htm
Here's a link to the Cherry page:
http://www.DigitalAmp.com/Cherry_amp2.htm
Here are links to reviews:
http://www.DigitalAmp.com/DAC_REVIEWS.htm
http://www.TrueDigitalPower.com/ (Amazon)
http://www.TrueDigitalAmp.com/ (Amazon)
Damping Factor
We received a question about damping factor. Both the DAC4800A and Cherry were tested for this at 1KHz, which is higher than normal for a damping factor test. Into 8 ohms, it measures around 200 (Rload/Rout). All the way down to 20Hz, the measurement was about the same.
There has been some controversy regarding the real meaning of damping factor, however. Tube amps are known for their sweet sound but usually have a low damping factor due to high output impedance. We believe the natural open loop performance of the amp comes into play as damping factor lowers. In most cases this occurs as frequency rises. With good power supply design and strong output stages, damping factor can mean less to the sound of an amp.
The proof is in the pudding, so there's no substitute for listening.
There has been some controversy regarding the real meaning of damping factor, however. Tube amps are known for their sweet sound but usually have a low damping factor due to high output impedance. We believe the natural open loop performance of the amp comes into play as damping factor lowers. In most cases this occurs as frequency rises. With good power supply design and strong output stages, damping factor can mean less to the sound of an amp.
The proof is in the pudding, so there's no substitute for listening.
Output Resistance Measurement Trick
Here's an easy measurement trick to find output resistance, but you need an accurate audio analyzer to use it:
Load the amp with 24 ohms and change the input voltage (sine wave) until you get exactly 24 volts out of the amp. Remove the load and subtract 24 from the new output voltage measurement. This is the output resistance in ohms! 24 ohms was picked to increase accuracy, but you can do the same thing with 8 ohms and 8 volts. The voltage measurement should have 3 digits after the decimal point for this to yield an accurate measurement.
Knowing the output resistance, you can calculate damping factor (DF = Rload/Rout). Note that output resistance can change over frequency.
Load the amp with 24 ohms and change the input voltage (sine wave) until you get exactly 24 volts out of the amp. Remove the load and subtract 24 from the new output voltage measurement. This is the output resistance in ohms! 24 ohms was picked to increase accuracy, but you can do the same thing with 8 ohms and 8 volts. The voltage measurement should have 3 digits after the decimal point for this to yield an accurate measurement.
Knowing the output resistance, you can calculate damping factor (DF = Rload/Rout). Note that output resistance can change over frequency.
Preamp Output Impedance and Amplifier Input Impedance
We received some questions about DAC4800A (and Cherry) input impedance, which is 10K. Here was our response:
It's simple... You get more noise with higher input impedance. Going from 10K to 100K might cost as much as 3dB, but since stock SNR is over 114dB, there's room. We don't do this stock because most preamps have a 600 ohm or lower output impedance, more than 10:1 (the typical "rule of thumb") to drive the amp. We opted for better SNR. The current of the preamp's output driving 10K is VERY low (microamps), so that's not really a significant factor. We can modify amps for 100K input impedance when requested, but there is some rework required.
We don't recommend preamps with AC coupled outputs, although these are few and far between. AC coupling between the preamp and amp creates a high pass filter that can rob you of low bass response.
It's simple... You get more noise with higher input impedance. Going from 10K to 100K might cost as much as 3dB, but since stock SNR is over 114dB, there's room. We don't do this stock because most preamps have a 600 ohm or lower output impedance, more than 10:1 (the typical "rule of thumb") to drive the amp. We opted for better SNR. The current of the preamp's output driving 10K is VERY low (microamps), so that's not really a significant factor. We can modify amps for 100K input impedance when requested, but there is some rework required.
We don't recommend preamps with AC coupled outputs, although these are few and far between. AC coupling between the preamp and amp creates a high pass filter that can rob you of low bass response.
A Few Words About Output Current
The output devices in the DAC4800A and Cherry amps are capable of pulsed drain current of more than 150A! However, the output stage cycle-by-cycle current limit is set to about 30A instantaneous.
Our Amps are NOT based on Ice Power Modules
We DON'T make amps with Ice Power modules! I replied recently to someone's "just another ice power amp" email comment. It takes a lot more amp design experience (20 years for me) to design high performance, high efficiency, high power amps from the ground up than to put off-the-shelf modules in a box with a power supply. Something like 90% of the digital amps out there are like that, so we're special. We designed the guts. We build the guts. We are the guts.
Digital Amplifier Company amps are built with proprietary patented technology that took more than 10 years to develop. It's a novel approach to modulation and control that allows closer-to-the-rails output swing while holding noise down as far as 120dB from max output. Really low THD is also possible with this technology, down to 0.0006% on the bench.
We are customer centric because we appreciate having the privilege of serving them. They are the reason we exist, and there's nothing quite like having customers repeatedly tell you that your design is the best they have ever heard. That makes all the hard work that goes into each amp well worth it. Thanks again.
Digital Amplifier Company amps are built with proprietary patented technology that took more than 10 years to develop. It's a novel approach to modulation and control that allows closer-to-the-rails output swing while holding noise down as far as 120dB from max output. Really low THD is also possible with this technology, down to 0.0006% on the bench.
We are customer centric because we appreciate having the privilege of serving them. They are the reason we exist, and there's nothing quite like having customers repeatedly tell you that your design is the best they have ever heard. That makes all the hard work that goes into each amp well worth it. Thanks again.
A Few Words About Buying Vintage Loudspeakers
Watch out for speakers with drivers you can't replace. Also, take a good look at the condition of woofer surrounds, dust caps, and the like. It may work well and sound good, but for how long? I burnt out one woofer each on two of my speaker systems, one DIY and the other a well known high end name, and couldn't replace the burnt drivers with exact replacements. One was luckily fixable with a "close replacement", and one lays dormant (Focal dual coil woofers). Of course, when one side blows out, you want to replace both sides. Crossovers are easier to find replacement parts for, that is, if the components are marked with the value (air coil inductors are notorious for no markings). Also check if the drivers are original. Good luck.
Friday, August 22, 2008
REAL ADVICE ABOUT PICKING AUDIO POWER AMPLIFIERS
I'm always weary of reviews from magazines. There are hardly any bad ones in fear of "burning bridges", getting sued, or losing a potential advertising client. No magazine in their right mind will admit to this. In fact they will deny it tooth and nail. Same deal for "product rewards"; many are bought and paid for indirectly.
Reviews that simply show measurements and no opinion whatsoever can usually be trusted, but are hard to find. Measurements can't tell you everything, but can tell you a lot. Manufacturers rarely falsify or embellish measurements due to fear of getting caught. However, there are some tricks that can make measurements look better, such as rating power at 10% THD, but this is also very rare. The most trustworthy source of how something sounds (beyond measurements) is actual customers. However, people have a tendency to like whatever they bought, so watch out for reviews that have no comparisons to anything else.
Regarding well recognized brand names, don't believe that just because a brand is well known and has a long history that they only make spectacular products. I had a WELL KNOWN (maybe the MOST well known amp brand of all) amp and was explaining my total disappointment with it to a friend. He said something like "I can't believe you bought one, everybody knows that amp sucks. It sounds just awful and was a complete failure for them". He also said that despite the bad sound of this design, they sold a TON of them.
One important figure is signal to noise ratio. Anything less than 100dB with respect to rated power should be avoided like the plague. There's no reason in this day and age that you can't design just about any type of amplifier at least that good. Noise is the enemy of good sound! If there is an SNR measurement at 1W, look for at least 85dB. This is A-weighted. Unweighted SNR is not as meaningful of a spec since the human ear has frequency dependent sensitivity curve. The "ear near the tweeter" test is actually quite useful if you have a single tweeter that's no more than 2" long in any direction. Listen not only to how much noise there is but also the character of the noise. Quality amps are dead quiet or close at idle.
Look for amps with THD+N below 0.01% for a good portion of the power curve and at all frequencies. This is especially important at low levels, so look for THD+N well under 0.1% at 1KHz, 100mW. Unfortunately, many manufacturers don't provide this spec, and if they do, it's probably on a graph (THD+N versus power), not in text, and sometimes you need to interpolate. Look for amps with direct coupled output. Output transformers are fine for distributed audio in your local supermarket sound system. They saturate at low frequencies.
Another one... Damping factor under 50 is a red flag. If damping factor or output impedance (reciprocal of this divided by the load impedance is damping factor) is not mentioned, it's probably poor. Also, as damping factor goes higher and higher it means less and less. I doubt any human being can hear the difference between an amp with a damping factor of 300 and one with a damping factor of 450 that's otherwise the same.
Regarding the "sound" as perceived by any reviewer comparing two or more amps, if the comparisons aren't double blind, they aren't super valuable. The mind simply can't remove bias reliably otherwise. Single blind is much more meaningful than non-blind. You might also be surprised how many so-called "golden ears" fail double blind listening tests (essentially can't tell the difference) when the amps are similar! It's funny to hear the excuses when you call them on it. This is especially amusing with aftermarket modifications. My theory is "buy something that was designed right in the first place".
Anyway... Regarding output power, watch out for manufacturers of anemic amplifiers that tell you their 30W amp is great for driving your 10-driver per channel tower speaker system. You need adequate power to drive your speakers through peaks unless you never "turn it up". Big speakers need big power to play loud and clean at the same time. I recommend a few hundred watts minimum per channel unless you're driving a pair of bookshelf speakers in a small room. Also watch for amps with lots of power but bad specs otherwise. Beware of amps with a spec called "instantaneous peak power" of something similar. This is technically twice RMS output power at rated output right before clipping. If you see this rating and it's NOT twice rated power, there's something fishy going on. This spec is used deceptively so beware of its use either way.
Regarding inputs, when possible, insist on balanced inputs or both unbalanced and balanced. Balanced audio is inherently better due to built-in noise cancellation, and if you can afford a good preamp with balanced outputs, use one for your primary audio system.
Here's something I wrote regarding pro amp measurements more than 3 years ago:
http://www.livesoundint.com/archives/2004/dec/power.pdf
Yes, I worked in pro sound for several years, but learned a lot. I know lots of high-end people think anything pro audio is junk. They are just wrong.
Reviews that simply show measurements and no opinion whatsoever can usually be trusted, but are hard to find. Measurements can't tell you everything, but can tell you a lot. Manufacturers rarely falsify or embellish measurements due to fear of getting caught. However, there are some tricks that can make measurements look better, such as rating power at 10% THD, but this is also very rare. The most trustworthy source of how something sounds (beyond measurements) is actual customers. However, people have a tendency to like whatever they bought, so watch out for reviews that have no comparisons to anything else.
Regarding well recognized brand names, don't believe that just because a brand is well known and has a long history that they only make spectacular products. I had a WELL KNOWN (maybe the MOST well known amp brand of all) amp and was explaining my total disappointment with it to a friend. He said something like "I can't believe you bought one, everybody knows that amp sucks. It sounds just awful and was a complete failure for them". He also said that despite the bad sound of this design, they sold a TON of them.
One important figure is signal to noise ratio. Anything less than 100dB with respect to rated power should be avoided like the plague. There's no reason in this day and age that you can't design just about any type of amplifier at least that good. Noise is the enemy of good sound! If there is an SNR measurement at 1W, look for at least 85dB. This is A-weighted. Unweighted SNR is not as meaningful of a spec since the human ear has frequency dependent sensitivity curve. The "ear near the tweeter" test is actually quite useful if you have a single tweeter that's no more than 2" long in any direction. Listen not only to how much noise there is but also the character of the noise. Quality amps are dead quiet or close at idle.
Look for amps with THD+N below 0.01% for a good portion of the power curve and at all frequencies. This is especially important at low levels, so look for THD+N well under 0.1% at 1KHz, 100mW. Unfortunately, many manufacturers don't provide this spec, and if they do, it's probably on a graph (THD+N versus power), not in text, and sometimes you need to interpolate. Look for amps with direct coupled output. Output transformers are fine for distributed audio in your local supermarket sound system. They saturate at low frequencies.
Another one... Damping factor under 50 is a red flag. If damping factor or output impedance (reciprocal of this divided by the load impedance is damping factor) is not mentioned, it's probably poor. Also, as damping factor goes higher and higher it means less and less. I doubt any human being can hear the difference between an amp with a damping factor of 300 and one with a damping factor of 450 that's otherwise the same.
Regarding the "sound" as perceived by any reviewer comparing two or more amps, if the comparisons aren't double blind, they aren't super valuable. The mind simply can't remove bias reliably otherwise. Single blind is much more meaningful than non-blind. You might also be surprised how many so-called "golden ears" fail double blind listening tests (essentially can't tell the difference) when the amps are similar! It's funny to hear the excuses when you call them on it. This is especially amusing with aftermarket modifications. My theory is "buy something that was designed right in the first place".
Anyway... Regarding output power, watch out for manufacturers of anemic amplifiers that tell you their 30W amp is great for driving your 10-driver per channel tower speaker system. You need adequate power to drive your speakers through peaks unless you never "turn it up". Big speakers need big power to play loud and clean at the same time. I recommend a few hundred watts minimum per channel unless you're driving a pair of bookshelf speakers in a small room. Also watch for amps with lots of power but bad specs otherwise. Beware of amps with a spec called "instantaneous peak power" of something similar. This is technically twice RMS output power at rated output right before clipping. If you see this rating and it's NOT twice rated power, there's something fishy going on. This spec is used deceptively so beware of its use either way.
Regarding inputs, when possible, insist on balanced inputs or both unbalanced and balanced. Balanced audio is inherently better due to built-in noise cancellation, and if you can afford a good preamp with balanced outputs, use one for your primary audio system.
Here's something I wrote regarding pro amp measurements more than 3 years ago:
http://www.livesoundint.com/archives/2004/dec/power.pdf
Yes, I worked in pro sound for several years, but learned a lot. I know lots of high-end people think anything pro audio is junk. They are just wrong.
Sunday, August 17, 2008
INTRO
I'm the founder of Digital Amplifier Company.
On our web site, we explain that we don't go to trade shows or pay for standard print or web advertising. Do you have any thoughts on this?
This post is to get feedback about our products. If you have auditioned or purchased either of our current products, please give us a shout.
This post is to get feedback about our products. If you have auditioned or purchased either of our current products, please give us a shout. The DAC4800A is a 1000W amp and the Cherry is a 1200W amp. They are both 2-channel units and have very respectable specs (<0.009% distortion, near 120dB SNR). Some have said that our amps are the best they have ever heard regardless of price. Reviews are posted on our site and on Amazon.com (we sell there too).
Please let us know if you have heard one of our amps or if you have visited our web site (www.DigitalAmp.com). I'd like to get some feedback ASAP, especially concerning comparisons with other high end amps. We plan to introduce some new stuff soon as well.
I'm offering my expertise regarding digital (Class-D) amps since I have been designing them since the mid 1980s. If you have questions about these designs, let me know, and I'll try and answer them.
If you are only a casual music listener who thinks low bit rate MP3s on your iPod are the best in audio, please refrain from commenting!
Thanks very much.
On our web site, we explain that we don't go to trade shows or pay for standard print or web advertising. Do you have any thoughts on this?
This post is to get feedback about our products. If you have auditioned or purchased either of our current products, please give us a shout.
This post is to get feedback about our products. If you have auditioned or purchased either of our current products, please give us a shout. The DAC4800A is a 1000W amp and the Cherry is a 1200W amp. They are both 2-channel units and have very respectable specs (<0.009% distortion, near 120dB SNR). Some have said that our amps are the best they have ever heard regardless of price. Reviews are posted on our site and on Amazon.com (we sell there too).
Please let us know if you have heard one of our amps or if you have visited our web site (www.DigitalAmp.com). I'd like to get some feedback ASAP, especially concerning comparisons with other high end amps. We plan to introduce some new stuff soon as well.
I'm offering my expertise regarding digital (Class-D) amps since I have been designing them since the mid 1980s. If you have questions about these designs, let me know, and I'll try and answer them.
If you are only a casual music listener who thinks low bit rate MP3s on your iPod are the best in audio, please refrain from commenting!
Thanks very much.
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